Super-paramagnetic nanoparticles are applicable in a variety of nano-bio fields including, in particular, MRI contrast agents, cell separation, hyperthermia, drug delivery, biosensors, etc., and are receiving a great amount of attention.
Methods of synthesizing super-paramagnetic nanoparticles include coprecipitation, hydrothermal synthesis, thermal decomposition, etc. Among these, coprecipitation and hydrothermal synthesis enable iron oxide nanoparticles to be easily manufactured by directly reacting iron (II) chloride and iron (III) chloride in an aqueous phase to precipitate them, but are problematic because the size of the nanoparticles is difficult to control. PCT/KR2005/004009 by Taeghwan Hyeon et al. discloses a thermal decomposition method in which a large amount of uniform nanoparticles are synthesized from a non-toxic metal salt, without size-selection procedure. However, this method is difficult to apply to bio fields because the nanoparticles having oleate attached to the surface thereof are difficult to disperse in an aqueous phase.
Methods of dispersing metal oxide nanoparticles having a hydrophobic ligand attached thereto in an aqueous phase include ligand exchange, preparation of self-assembled nanoparticles using an amphiphilic material, encapsulation using a polymer, etc.
Typically exemplified by Feridex available from AMAG or Resovist available from Bayer-Schering, a conventional commercially available liver-specific contrast agent comprising super-paramagnetic nanoparticles is manufactured using coprecipitation, but has problems in that it is difficult to control the size of iron oxide nanoparticles that exhibit contrast enhancement and the size distribution is not uniform, undesirably making it difficult to increase the contrast of the contrast agent. Furthermore, it is not easy to adjust the shape and the size of capsules having iron oxide encapsulated therein, making it difficult to regulate the biodistribution upon intravenous injection.
Korean Patent No. 10-2006-0021536 by Cheon Jin-Woo et al. discloses a technique for manufacturing water-soluble nanoparticles in which iron oxide nanoparticles synthesized using thermal decomposition are stabilized with a multifunctional ligand. According to this method, the ligand comprising an adhesive region, a cross-linking region, and an reactive region is exchanged with oleate to individually coat the iron oxide nanoparticles, and thus the size of the nanoparticles is difficult to control as required to adapt them to a liver-specific contrast agent and the aggregation effects of iron oxide nanoparticles that increase T2 relaxivity cannot be expected.
Korean Patent No. 0819377 by Ham Seung-Joo et al. discloses the use of nanoparticles synthesized via thermal decomposition as a contrast agent by dispersing them in water using an amphiphilic compound. This method collects the magnetic nanoparticles with the amphiphilic compound having hydrophobic-hydrophilic portions and thus the material is limited to the amphiphilic compound, and the size of nanoparticles, size distribution, encapsulation efficiency of iron oxide nanoparticles, etc., required as the contrast agent, are difficult to control, making it difficult to enhance T2 relaxation.
Poly(lactide-co-glycolide) (PLGA) which is a copolymer of lactic acid and glycolic acid is a material approved as an injectable material by the FDA thanks to excellent biocompatibility and biodegradability, and has recently been adopted to a variety of drug delivery fields and medical applications. PLGA causes hydrolysis with surrounding water molecules and thus decomposes with breaking an ester linkage, and thereby is decomposed within ones of months and easily eliminated from a living body.
The present invention pertains to an MRI contrast agent for diagnosing a liver in high contrast, and more particularly to a method of encapsulating iron oxide nanoparticles coated with oleic acid using a biodegradable polymer to have a uniform size and the use thereof as an liver-specific contrast agent for MRI.
KR0702671 filed in 2005 by the professor Kim Jong-Duk of KAIST discloses a method of encapsulating iron oxide nanoparticles via an emulsification-diffusion method using a biodegradable polymer such as PLA, PGA, PLGA or the like, in which iron oxide nanoparticles resulting from coprecipitation are adopted. Hence, the iron oxide nanoparticles are provided in the form of oleate not being attached to the surface thereof, and a different encapsulation method is applied thereto. Also, in the case where the maximum encapsulation efficiency exceeds 5 wt %, the size of capsules increases due to aggregation of nanoparticles, and cannot be adjusted.
A paper (J. Nanosci. Nanotechnol., 9, 7118-7122 (2009)) published in 2009 by B. S. Jeon et al. discloses a method of maintaining the size of nanoparticles while increasing the encapsulation efficiency of emulsification-diffusion method by using oleic acid, dodecanoic acid, octanoic acid or the like as an organic acid that attaches itself to the surface of nanoparticles, but the maximum encapsulation efficiency is limited to 7 wt %.
As mentioned above, commercially available liver-specific contrast agents use super-paramagnetic iron oxide nanoparticles synthesized by coprecipitation technique, and iron oxide nanoparticles recently synthesized by thermal decomposition have high crystallinity and uniformity and thus may exhibit high magnetization and high MR contrast enhancement; however, in the manufacturing process, these nanoparticles having hydrophobic oleate attached to the surface thereof are difficult to disperse in an aqueous solution, making it difficult to adopt them to bio biomedical applications.
Particularly in research into biomedical applications, because pharmacokinetics of an intravenous injection of nanoparticles may greatly vary depending on physical and chemical properties such as overall particle size, size distribution, zeta potential, density, etc., there is a need for techniques that accurately adjust these as desired.
In order to disperse the iron oxide nanoparticles having oleate attached thereto in an aqueous solution, various methods using ligand exchange or using polymeric micelles, liposomes, dendrimers, etc. by means of an amphiphilic material are under study, but MR contrast enhancement is not sufficient and the particle size is difficult to control, and thus pharmacokinetics of particles becomes inconsistent.
Also the dispersion of iron oxide nanoparticles may include for example an emulsification-diffusion method, in which when two immiscible phases (O/W) are forcibly emulsified and the organic solvent diffuses into the water phase due to a difference in concentration, an active material and a polymer dissolved in the organic solvent are diffused together, thus forming polymer capsules. However, this method is disadvantageous in terms of low encapsulation efficiency because it is difficult to disperse the iron oxide nanoparticles having oleate attached thereto in a partially water-miscible solvent.
In particular, in the case where the encapsulation efficiency is lower than 0.5 wt %, the preparation of a 1 mg Fe/ml contrast agent requires a concentration of 200 mg capsule/ml or more. As the osmotic pressure and the viscosity of an injection solution are higher, the injection may cause pain, or in severe cases, osmotic shock may ensue.
Furthermore, upon injection into a living body, microcapsules larger than nanocapsules may block capillary vessels undesirably causing side effects such as necrosis. Hence, it is very important to control the uniformity of nanocapsules.